As a heat sink essential for cooling a heat generating body, such as a high-output semiconductor laser element, a heat sink having a structure, in which a fluid channel is formed by joining copper members or other metal members and in which a fluid for cooling the heat generating body is circulated inside the fluid channel, is known.
To cool semiconductor laser elements, an arrangement, in which semiconductor laser elements are mounted and layered so as to be respectively sandwiched by heat sinks as shown in FIG. 5, is employed. Here, since the respective heat sinks not only cool the respective semiconductor laser elements but also serve as electricity conducting paths among the respective semiconductor laser elements, a potential difference arises between the above-described fluid and the heat sinks.
Thus when long-term circulation of approximately 10,000 hours is performed, volume reduction occurs at the anode side of each heat sink and deposition and attachment of copper corrosion products occur at the cathode side due to electrochemical mechanisms at liquid-contacting portions as shown in FIG. 7.
Such corrosion can be restrained by lowering the conductivity of the fluid. However, as the conductivity is lowered (that is, as the resistance is increased), the activity is increased and the corrosive property thus heightens. Also since a device for lowering the conductivity leads to large size and high cost, the lowering of the conductivity of the fluid is not suitable for preventing corrosion. Also since leak currents flow even if the conductivity is lowered, corrosion effects will occur if circulation is performed over a period of the order of several years (several tens of thousand hours of actual operation).
To prevent such a corrosion, the forming of an isolating layer, which prevents the contacting of the inner wall surfaces of the metal members with the fluid, on the inner wall surfaces without degrading the thermal characteristics has been tried. For example, in a Patent Document 1 described below, a metal, such as Au or an Au alloy, is used as the isolating layer. In a Patent Document 2 described below, an isolating layer formed of resin is used as the isolating layer of a metal tube. The inner wall surface of the obtained metal tube is coated by the resin isolating layer in advance.
Patent Document 1: Japanese Published Unexamined Patent Application No. 2003-273441
Patent Document 2: Japanese Published Unexamined Patent Application No. Hei 5-118473